Pantheon Dome

The Pantheon in Rome is the best-preserved major building of ancient Rome, and its dome is one of the most important roofs in history. Built of unreinforced concrete, it has stood for nearly 1,900 years and remains the largest dome of its kind.

Date

The present building was completed under Emperor Hadrian and probably dedicated around 126 CE. Sources give slightly different construction ranges: about 113-125 CE, with work possibly begun under Trajan, or 118-128 CE in Britannica. An earlier temple on the site was begun by Agrippa in 27 BCE; it was destroyed by fire, and the present rotunda and dome are a complete rebuilding.

The dome

• Interior diameter: about 43.3 m (142 ft), equal to 150 Roman feet. The height from floor to the top of the dome is the same, so the interior would contain a perfect sphere.
• The oculus, an open circle at the top, is about 9.1 m (30 ft) across and is the only source of daylight.
• The shell is about 6.4 m (21 ft) thick at its base, thinning to about 1.2 m (3.9 ft) around the oculus.
• The inner surface has five rings of 28 sunken coffers, which reduce weight.

How it stands

Roman builders graded the concrete by height. The lowest, thickest part uses heavy travertine aggregate, the middle uses broken terracotta tile, and the top uses light tufa and pumice. The oculus removes weight where it would cause the most stress. Unlike most surviving ancient domes, the Pantheon needs no tie rods, chains or later reinforcement.

The roof covering

The dome's exterior was once covered with bronze tiles, described in some sources as gilded; it was not a copper roof. In July 663 the Byzantine emperor Constans II visited Rome, stripped the tiles and sent them to Constantinople. The roof stayed uncovered for nearly a century until Pope Gregory III (731-741) had it lined with lead sheets.

Survival

The building was consecrated as a church, St Mary and the Martyrs, on 13 May 609, and continuous use helped preserve it. It remained a reference for later dome builders; the oculus at the top of St Paul's Cathedral in London, for example, was inspired by it.